In recent years, fiber-reinforced composite materials using reinforcing fibers such as carbon fibers and aramid fibers have been used as structural materials of aircraft and motor vehicles, and for sports applications as tennis rackets, golf shafts and fishing rods, general industrial applications and the like owing to the high specific strength and specific elastic modulus thereof. Methods for producing fiber-reinforced composite materials include a method of using a prepreg as an intermediate sheet-like material in which reinforcing fibers are impregnated with an uncured matrix resin, laminating multiple plies of the prepreg and subsequently heating for curing, and a resin transfer molding method of pouring a liquid resin into the reinforcing fibers disposed in a mold and subsequently heating the resin for curing.
Among these production methods, the method of using a prepreg has an advantage that a fiber-reinforced composite material with high performance can be easily obtained for such reasons that the orientation of reinforcing fibers can be strictly controlled and that the degree of freedom in designing a laminate configuration is high. As the matrix resins used in the prepregs, thermosetting resins are mainly used in view of heat resistance and productivity, among them, epoxy resins are suitably used in view of the adhesiveness between the resin and the reinforcing fibers, dimensional stability, and mechanical properties such as strength and stiffness of the composite material obtained.
Hitherto as methods for enhancing the toughness of an epoxy resin, for example, methods of mixing a rubber ingredient or thermoplastic resin excellent in toughness for forming a phase-separated structure together with an epoxy resin have been tried. However, these methods have such problems as the decline of elastic modulus or heat resistance, the deterioration of processability due to viscosity rise and the decline of appearance quality due to void formation, etc. For example, a method for greatly enhancing the toughness by adding large amounts of a phenoxy resin and polyethersulfone, to cause phase separation thereof is proposed (Patent Document 1). However, since the mixing of a thermoplastic resin exerts large influence of viscosity rise, the processability tends to deteriorate. Especially in the case where the epoxy resin composition is used for producing the prepregs for the primary structural materials of aircraft, the mixed amount of the thermoplastic resin must be decreased to avoid the adverse effect on processability, and there is a trend of being unable to make the epoxy resin sufficiently tough.
On the contrary, in recent years, methods of enhancing the toughness and impact resistance by using a diblock or triblock copolymer for forming a nanosized phase-separated structure can be seen. For example, Patent Documents 2 to 5 propose methods of enhancing the toughness by using a styrene-butadiene copolymer, styrene-butadiene-methacrylic acid copolymer or butadiene-methacrylic acid copolymer in combination with a specific epoxy resin. However, the cured resins obtained by these methods are insufficient in the heat resistance and elastic modulus for aircraft applications.
For enhancing the toughness, a technique of adjusting the component ratio of an epoxy resin composition and controlling the phase-separated structure of a block copolymer, to thereby enhance the toughness is proposed (Patent Document 6). Further, a technique of using an amine type epoxy resin with a high crosslinking degree so as to obtain an epoxy resin composition with a high elastic modulus, high heat resistance and high toughness is proposed (Patent Document 7). These techniques are especially effective for applications such as golf shafts requiring both torsion strength and impact resistance. On the other hand, for applications such as bicycle frames and bats requiring higher impact resistance, the impact resistance of the composite materials tends to be insufficient. Moreover, the use for aircraft applications requiring high heat resistance has been difficult.    Patent Document 1: JP 2007-314753 A    Patent Document 2: WO 2006/077153    Patent Document 3: JP 2008-527718 A    Patent Document 4: JP 2007-154160 A    Patent Document 5: JP 2008-007682 A    Patent Document 6: WO 2008/001705    Patent Document 7: WO 2008/143044